Wireless microservers

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  • 1. 1. INTRODUCTION: With Bluetooth components getting smaller and cheaper, we might soon integratewireless Microservers into all kinds of electronic devices. The authors explore applying ageneral-purpose, pluggable microserver, based on wireless application protocol and Bluetoothtechnology, for remote control purposes. Since the early days of the Web, server-side executable content has been an importantingredient of server technology. It has turned simple hypertext retrieval into real applications.Not surprisingly, the idea of remotely controlling devices through the Web1,2 has alwaysseemed near at hand. Because hypertext user interfaces can run on any Web browser, UIdevelopment boils down to Web content creation. Furthermore, thanks to the HTTP standard’ssmart and scalable nature, we can fit embedded servers into simple 8-bit microcontrollers withonly a few Kbytes of RAM and ROM (see the ―Embedding Servers into Devices‖ sidebar).3Ever since we started integrating hypertext browsers into mobile phones, people have proposedusing mobile phones as remote controls. Now, with the provision of short-range wirelessconnectivity— for example, through Bluetooth— mobile phones and other handhelds mightsubstantially change the way people interact with electronic devices. Here, we report on oureffort to create a low-power wireless microserver with a very small form factor and connect it tomobile devices using standard consumer technology. 1
  • 2. 2. CANDIDATE APPLICATIONS AND TECHNOLOGIES: Consumer electronics have used wireless lowcostremote controls for decades. Addingembedded servers to devices will create a new range of use cases beyond the limited capabilitiesof infraredRemotes:• Browsers could then allow real interaction instead of just sending one-way commands such asInfrared remotes.• We could harmonize similar operations in terms of a Web UI, even if the built-in UIs of theparticular devices differ.4 A good example is setting a device’s clock, which is a commonoperation in many consumer devices but always requires a unique—and often error-prone—Implementation.• We could distribute the UI among the device’s built-in UI and the handheld’s UI.4 Forexample, we could export parental control functions—such as ―edit play-time budget or gametype‖—of game consoles to a mobile phone.• A server with memory could personalize its UI and service by collecting and interpreting itsown usage patterns and those of adjacent servers.• Devices that lack a UI, have a restricted UI, or are purposely hidden could export a UI to ahandheld.• While the handheld is connected to the device, the device could connect to the Internet, in casethe handheld supports cellular data calls. Eventually, we could turn each compatible handheld into a general-purpose remotecontrol. In a joint project, Nokia and the University of 2
  • 3. 3. EMBEDDING SERVERS INTO DEVICES: There are several ways to make things visible on the Web. In the simplest case, a serverhosts an item’s Web presence without a physical connection to the item. A handheld device readslinks between the item and its Web presence, connects to the respective URL, and retrievesinformation about the item. A well-known example for this approach is the Cooltown Museum,1where small infrared transceivers are located close to the pictures. When coming close, thevisitor’s PDAs receive Web links that point to the information pages for the particular picture.Unfortunately, interacting with the item itself is impossible. Interaction with a device would be possible if the device had a wireless control interfaceto its internal logic. For example, the mobile terminal could download a device-specific userinterface application from the Web and use it to control the device through a device-dependentprotocol (see Figure A1). This approach might become feasible when we can download Javaapplications into mobile terminals with access to Bluetooth APIs. Accessing the deviceimmediately and locally without an Internet connection would be possible only if the devicecontained an embedded Web server (see Figure A2). An execution environment, such as serverside scripting, would be required to interact with the device’s logic. Short-range connectivityseems to be an obstacle, but it empowers location-aware applications through the wireless link’slimited reach. If a user wants to adjust a microserver-equipped TV’s volume, he or she does notwant to accidentally interact with somebody else’s TV. Therefore, short-range wireless radiolinks, preferably using unlicensed bands, are well suited for networking things and people. 3
  • 4. Figure A. (1) UI application (such as Java Midlet) is downloaded and controls the device by acontrol protocol. The internal and wireless link for the control protocol must be bridged. (2) Thedevice hosts the server comprising UI and execution environment Dortmund created a low-power wireless microserver. (We use the term microserver torefer to small and cost-efficient embedded Web server implementations that are either integratedor plugged directly into the device.) Because mobile phones have a much higher population thanPDAs and will likely be the first truly ubiquitous computing devices, we chose to connect themicroserver to a mobile phone. Although there are many technologies available for embedded middleware for distributedcomputing and service discovery, such as Java, Jini, and UPnP,5 we decided not to use suchmiddleware components. For our project, these technologies have two important drawbacks: theyare not widely applied today and their implementation takes more processing resources then wewere willing to spend. 4
  • 5. For example, a small implementation of Jini that doesn’t even use Java-RMI wouldrequire approximately 100 Kbytes of ROM and 50 Kbytes of RAM plus another 100 Kbytes ofRAM and 70 Kbytes of ROM for a lean Java virtual machine that includes basic packages. Wechose to use Bluetooth because it is the only short-range radio technology currently deployed inmobile phones. Our implementation fit into the free memory of a commercial Bluetooth module,which was about 40 Kbytes of ROM and 4 Kbytes of RAM. For the phone, we used existingimplementations of the browser and Bluetooth software without adding a new middlewarecomponent. For remote method invocation, we based our solution (discussed later) on thepopular yet old-fashioned common gateway interface. 5
  • 6. 4. WAP AND BLUETOOTH The wireless application protocol is an industry-wide standard to connect mobilePhones to the Web. It is designed especially for the mobile phone environment and its limitedbattery power and memory, small display, and low transmission rates. Similar to the Japanese I-mode, WAP provides Web access for mobile devices. Usually, browsers directly request HTMLcontent from a Web server using HTTP. A typical WAP infrastructure also needs HTTP (seeFigure 1), but the WAP protocol stack (see Figure 2) and the WML (Wireless Markup Language)are tailored for the limited transmission capacity and resources of mobile devices. In addition,unlike HTTP, WAP allows server push operation, so the server can send a WML page to abrowser without a request.Figure 1. A wireless application protocol request and response model. A WAP request is directedto a WAP gateway. The gateway decodes the request and transforms it into an HTTP request foran ordinary Web server. The gateway then encodes the Wireless Markup Language pageresponse from the HTTP server and sends it as a WAP response back to the requesting WAPdevice. The WAP standard also specifies different levels of security that enable data encryptionand trusted communication—for example, for electronic payments or banking applications. 6
  • 7. Because future mobile phones will have larger screens and more processing power, thenext-generation WAP standard (WAP 2.0) will support the HTTP protocol and XHTML as amarkup language. Another reason for this support is the general trend toward all-Internetprotocol (IP) infrastructures. Currently, most Web browsers available in Europe are based onWAP 1.1, which we selected for our implementation. However, you could apply our concept toother mobile Web technologies, such as I-mode and WAP 2.0 without a substantial change incomplexity. Bluetooth is an industry standard for short-range, low-power, wirelesscommunications and networking. com/dev/specifications.asp). It uses radio transmission in thelicense-free band of 2.4 GHz and can transmit voice and data with bit rates up to 720 kbits/secwithin approximately 10 meters. Bluetooth technology supports pointto- point and point-to-multipoint connections. We can actively connect a Bluetooth device to seven devicessimultaneously. These devices build a so-called piconet, and every piconet contains up to sevenslaves and is controlled by a master. Several piconets can be linked together to form a scatternet.Bluetooth was not envisaged to just replace cables with interconnecting mobile phones, PCs, andperipheral devices such as headsets or printers. It aims to let arbitrary electronic devices form adhoc networks to jointly advertise and use each other’s services. It specifies its own servicediscovery protocol, and every Bluetooth device usually implements its own service discoveryserver and client.GLOSSARY:API: Application programmers’ interface BCU: Bus Coupling UnitCDMA: Code Division Multiple Access DUN: Dial-up networkingEIB: European Installation Bus GPP: General purpose portGPRS: General packet radio service IP: Internet protocolLAP: LAN access profiles PAN: Personal area networkingPEI: Physical external interface UI: User interfaceWDP: Wireless datagram protocol WAP: Wireless application protocolWML: Wireless Markup Language WSP: Wireless session protocolUART: Universal asynchronous receiver transmitter 7
  • 8. Figure 2. A WAP-over-Bluetooth protocol stack. Either the LAN access profile or the personalarea networking profile is used for Internet protocol adaptation .Figure 3. Consecutive screenshots of a mobile phone (a) while a user actively connects to avending machine and (b) when the server initiates connection (the connection request—―Connectto MP3 player?‖—comes without user interaction). 8
  • 9. The Bluetooth community has identified many application areas for Bluetooth. To assureapplication-level interoperability between devices of different manufacturers, it is not sufficientto specify the Bluetooth technology; we also need to agree on the technology’s use by higher-level protocols and applications. Bluetooth profiles address these aspects.4.1 WAP over Bluetooth: The WAP protocol stack includes at the lowest level the wireless datagram protocol(WDP). This layer implements the bearer adaptation and is defined for a variety of bearers, suchas GSM, Code Division Multiple Access, and the general packet radio service. The WDP doesn’tcover Bluetooth but it does cover IP. In fact, WDP is identical to the well-known user datagramprotocol, in case IP is the bearer. So the challenge is to define an IP encapsulation protocol forBluetooth, which has been done in both the LAN access profile and the personal area networking(PAN) profile (see Figure 2). The LAP makes Bluetooth behave like a standard serial port thatlegacy software and devices can use. Consequently, the LAP uses the same mechanisms as aterminal that accesses a network over a serial connection, namely point-topoint protocol and IP.When using the PAN profile, Bluetooth behaves like a direct LAN connection: the BluetoothNetwork Encapsulation Protocol makes Bluetooth behave similar to Ethernet using the Bluetoothdevice address as a hardware address. The PAN profile implements a lighter stack than the LAP.The LAP is already approved in the Bluetooth 1.1 specification, but future implementationsmight favor PAN, because it doesn’t require serial port emulation and point-to-point protocol.4.2 WAP-over-Bluetooth user experience: At first glance, Bluetooth is just another bearer for an existing service. After weestablished the Bluetooth and WAP connection between the client and server, browser operationis the same. However, there are some major differences in the WAP user experience betweencellular and Bluetooth connections. The first difference concerns location awareness. Bluetooth’slimited range is not necessarily a disadvantage. A WAP-over- Bluetooth server knows that oncea user connects, he or she is close. Thus, it can offer location-aware services, and servers indifferent places can reuse the same URLs. 9
  • 10. A second difference is that bandwidth is typically higher in Bluetooth than in cellularsystems, allowing richer content and better performance. Finally, there are two scenarios forestablishing a connection. In the first scenario, the user (WAP client) initiates the connection (seeFigure 3a). Upon user request, the WAP client searches for all Bluetooth devices in the user’sproximity, then creates a WAP connection to the selected WAP service. The user might select afunction such as ―search for Bluetooth devices.‖ Subsequently, the phone displays the Bluetoothdevices found. The user selects the desired WAP service and the phone displays this server’sstarting page. In the second scenario, the WAP gateway initiates the connection (see Figure 3b).In this case, we assume the WAP client is in ―discoverable‖ mode—that is, other Bluetoothdevices can find it. Once the user terminal comes into the gateway’s proximity, the WAPgateway actively connects and displays its starting page on the user’s phone. It is either animplementation choice or a user option whether to confirm the connection. In both examples, weassume that neither the client nor server require authentication. 10
  • 11. 5. IMPLEMENTATION OPTIONS FOR EMBEDDED WAP SERVERS: If we leave out the WAP standard’s optional features and merge the server withFigure 4. (a) An embedded WAP server versus (b) a pluggable server composed of a standardconnector and a standard control interface.Figure 5. (a) A preprogrammed UI versus (b) a UI downloaded from the device.the gateway, we can implement an embedded WAP-over-Bluetooth server in an 8-bitmicrocontroller using a few Kbytes of RAM and Flash, provided that Bluetooth (including thestack) is running on a dedicated component. Such optional features mainly include the WAPsecurity layer and the transaction layer (see Figure 2). 11
  • 12. Bluetooth implements flow control, retransmission of lost packets, authentication, andencryption—thus replacing some of the sacrificed WAP features. Because the embedded serverdoes not require an internal separation between the WAP gateway and HTTP server, we candirectly store the content in the final WAPencoded message format, thus making WAP encodingand decoding obsolete. Figure 4a depicts a simple model of such an embedded WAP server.Typically, the WAPserver would either run on the baseband processor inside a Bluetooth moduleor on a host processor connected to the Bluetooth module. The interfaces include general-purpose ports, a universal asynchronous receiver transmitter (UART), and the like. Logically,part of the WAP UI would be implemented as server-side scripts, triggering actions on userinteraction. The concept of embedded WAP servers is appealing at first glance. Any mobilephone can act as a remote control for Bluetooth devices, which simply export their UIs in astandard hypertext language. However, once these servers are embedded in low-cost devices,they can unduly increase the devices’ costs. The server’s cost amounts to at least the cost of theBluetooth module, including external circuitry. This can be 10 Euros and more, and will remain so at least until mid 2003. Customersusing the embedded server feature might be willing to pay more, but many people might end uppaying for a feature they never use. Moreover, Bluetooth technology and hypertext UIdevelopment is clearly outside the core competence of many device manufacturers. Instead ofembedding the server into the device, we could provide a robust and cheap standard connectorwhere a pluggable server can be retrofitted (see Figure 4b).6 Such a connector would, forexample, provide several general-purpose ports and a UART option. Unlike embedded servers,we could cost-efficiently manufacture general-purpose pluggable servers in large quantities.Moreover, if we could separate the UI content from the corresponding devices and pluggableservers, then independent marketplaces for UIs, servers, and devices could emerge. We couldmove pluggable servers from one device to another—for example, if a device is not used forsome time, we could remove the server and plug it into some other device. This concept could,for instance, be interesting for toy construction kits (such as Lego), which increasingly usedigital technology to construct sophisticated machines, vehicles, and robots. 12
  • 13. Because these kits consist entirely of pluggable components, a pluggable server would bea natural extension. Eventually, we’ll have to decide whether to use an embedded or pluggableserver on a case-by-case basis. However, the question remains, ―How do we get the UIinto theserver if we purchased the server independent of the device?‖Figure 6. Downloading a UI over a network.Figure 7. Downloading a UI over a network (method 4).5.1 Distributing the UI: We must either preload general-purpose, pluggable servers when selecting a candidate UIor externally load the UI when we first plug it in. In addition, for built-in servers, we’ll want tobe able to remotely update UI content. Here, we present four methods for provisioning UIs. 13
  • 14. We borrow our first method, which applies to pluggable servers, from generalpurpose IRremote controls: Every server can be preprogrammed for many devices. Once the server isplugged into a device, the device identifies itself to the server (either on startup or on thedevice’s request) and the server selects the corresponding UI from several available UIs (seeFigure 5a). This method is not very flexible and adds undue cost to the server because it reservesmemory for UIs that a particular customer might never use. More desirable are servers thatdownload the desired UI from some source. This method assumes that the device simply storesits UI in memory (for example, ROM) and uploads it to the server once it is plugged in (seeFigure 5b).6 Compared to the previous method, the total cost is lower, because only the requiredUI must be stored in memory. However, it makes the device slightly more expensive, and theserver—but not the UI—is provided separately from the device. Moreover, we can’t easilyupdate the UI. Obviously, the most flexible method is to download the UI over a network (seeFigure 6). One simple implementation is to connect the server to a PC that is connected to theInternet. The PC downloads the UI file from the manufacturer’s Web site and then stores it in thepluggable or built-in server. The fourth and most convenient method assumes that the server creates a Bluetooth dial-up network connection to the UI server through a Bluetooth phone. Advantageously, this phonewould be the same phone that is used to access the server.This method consists of several steps (see Figure 7):1. The user plugs the server into the device and creates a Bluetooth connection to it.2. The server retrieves the device ID and manufacturer URL from the device.3. The server discovers that the UI is not in memory. It creates a dial-up network connection toan ISP and downloads the UI from the manufacturer or som service provider. Alternatively, thepluggable server directly dials up the manufacturer. Dial-up networkingover Bluetooth issupported by the dial-up networking profile.4. After the server has downloaded the UI, it presents the UI starting page to the user. 14
  • 15. Clearly, the UI does not have to be prestored in the server, and no other devices (such asa PC) are needed for the download. This procedure could also be useful for regularly updatingthe UI or downloading informative pages into the server (perhaps to advertise the device’s newmodel). Only when the server is plugged into a device for the first time will the user have to waituntil the download completes. However, the server could cache the UIs of recently connecteddevices to avoid delays when being moved from one device to another. Typical WML pagesoccupy only a few hundred bytes of memory. A simple method invocation procedure throughCGI scripts We cannot realize remote control applications without server-side scripting, becauseuser interaction must trigger I/O operations on the server’s interface to the device’s logic. Thechallenge is to achieve some basic means for invoking device functions without using distributedcomputing middleware such as Jini or UPnP.Figure 8. (a) Microserver prototype implementation and (b) the most important blocks of thesoftware architecture. 15
  • 16. would set Port1 to one, read a string from the serial port, store the string into the browservariable my String, and load URL myURL.wmlc. (The actual syntax of the implemented script isdifferent. This example just explains the idea.) Besides reading and writing to the interfaces, theimplemented script can also increase and decrease counters and read and write string tokens (likecookies) to the server’s flash memory. This approach is rather primitive, but it works fine formany applications without requiring custom server scripts. We implemented the script in nativecode as part of the server and did not require any script engine for execution. The ROM usagewas 3 Kbytes.5.2 WAP-OVER-BLUETOOTH DEMONSTRATOR IMPLEMENTATIONS: We developed two working implementations of the WAP-over-Bluetooth server—a PCreference implementation using a Bluetooth PC card and an embedded implementation. Figure 8shows the server hardware (the dimensions are 32 × 40 mm), with the server implemented insidea Bluetooth module. The server’s total footprint is about 4 or 5 Kbytes of RAM and 35 or 40Kbytes of ROM, depending on the profile used (LAP or PAN). This includes a connectionlessWAP stack without WAP security. The connector implements two UARTS, several I/O ports,and a power supply. We can update the server’s content and software through a PC applicationbut not yet over server-initiated dial-up networking. We also modified a commercial Bluetoothphone’s software to support WAP-over-Bluetooth. The first ideas many people had were aboutremote-controlled PC applications, even though these are not embedded applications. Weimplemented a WAP application to control PowerPoint presentations using a Bluetooth phone.Another interesting demo application we did was to jointly edit an MP3 player’s play list(running on a Bluetooth-equipped laptop) for clubs and private parties. Interestingly, these kindsof demos were rather easy to implement once the server platform was available. Two studentswere able to do both the PowerPoint and MP3 demos in just a few days. However, theimplementations that used the embedded microserver hardware were the most interesting. 16
  • 17. 6. APPLICTIONS:6.1 THE TOY CRANE DEMONSTRATOR: The toy crane demonstrator is a typical candidate for a pluggable server. We bought a toycrane and removed the cable remote to connect the motors to relays. We then connected theserelays to a Microservers general- purpose ports, including the corresponding UI.Figure 9. The toy crane demonstrator.The server is located in the driver’s cab and connectedthrough relays to the crane’s motors. The (a) current WAP menu displayed is used to (b) lift andlower the crane’s boom. 17
  • 18. Figure 10. (a) The installation bus provides a network between devices and controls; (b) alldevices and controls are connected via a Bus Coupling Unit (BCU). We purposely selected the toy crane to see whether a WAP UI could sufficiently replacethe powerful joystick operation. We showed the crane at trade shows and to the Bluetoothcongress. Users familiar with WAP browsers could operate the crane intuitively without majorinstructions. We displayed the links as small icons representing individual control options. Thephone UI didn’t allow any proportional control (similar to a joystick), but we implemented theWAP content (see Figure 9) such that users could repeatedly start and stop a selected movement(for example, ―lower the boom‖) by pressing the same button. The delay between key pressesand motor activation was negligible. 18
  • 19. 6.2 HOME AUTOMATION: For the next demonstrator, we connected a bluetooth server to a home bus. The goal wasto operate and control devices on the home bus via Bluetooth phones. We implemented aworking version of the demo using the PC reference implementation. The most widely deployedinstallation bus for home automation in Europe is the European Installation Bus. An installation bus system aims to decouple network control and AC power distribution.This is achieved by providing two logically independent networks: first, the 110/220V powerdistribution network, and second, a low-voltage network for control purposes. Alternatively, thecontrol information could be transmitted through the power distribution network usingmodulation techniques, which saves extra wiring (a ―power line EIB‖). The advantage ofseparating power distribution and control is that the connection between control points (switches,dimmers, and control pads) and devices is not hardwired but can be configured. Similar to acomputer network, datagrams are sent between control points and devices to trigger the desiredactions in the devices—like switching a light on and off. Every control point (switch, button,control pad, and so forth) can be set up to control every device or clusters of devices (see Figure10a). In the simplest case, a light switch is configured to switch a particular lamp on or off. A PCor a dedicated EIB device connected to the installation bus configures the network. EIB controlpoints usually consist of two components. The main part, normally not visible to the user, is theBus Coupling Unit (see Figure 10b). The second part, the application module, contains the UIand the control logic. This could be a simple light switch but also a heating regulator. Theapplication unit is plugged directly into the BCU’s physical external interface (PEI) and thushides the BCU in the wall. EIB bus systems are widely deployed, mainly in office buildings andhospitals but also in houses. Although these systems provide some convenience, they still sufferfrom the fact that the UI to the home network is largely unchanged compared to plain oldinstallations: switches, dimmers, and so forth. Here the idea of the pluggable server comes into play: Every BCU can send controldatagrams to any device connected to the installation bus. Logically, we could go into an EIB 19
  • 20. networked building, unplug an arbitrary application module, and plug a WAP-over-Bluetoothmicroserver into the BCU using the PEI interface (see Figure 11). Once the microserver is loadedwith the correct content, a user can control any device connected to the installation bus throughhis or her Bluetooth phone’s WAP browser. For our demonstrator, we connected a Bluetooth-equipped laptop to the BCU. In Cooltown terms, this could be called a placeManager, 7 becauseit provides a Web presence for the devices and organizes them in its Web presentation.Figure 11. Controlling the home network via a plugged-in WAP-over- Bluetooth server. To ease content creation, we created a software tool that automatically converts thenetwork configuration data into server content. The configuration data is available as a binaryfile, which is created by the commercial EIB software tool used to configure the installation bus.The most interesting lesson learned from this demonstrator is that, in some areas, deployingpervasive computing applications does not suffer from the famous henand- egg problem. Theseinstallation busses, for example, are deployed today and a small additional investment for thepluggable server turns such a building into an attentive environment. Typical installation bussesalso contain sensors such as light meters and thermometers that might provide useful contextinformation. It would be interesting to study how to make the WAP UI context aware, alsotaking into account personal usage patterns. 20
  • 21. 7. CONCLUSION: Future implementations of both mobile and embedded servers will be more powerful andthus will use true distributed computing middleware. However, for now, we have shown thatconnecting electronic devices to the Web with inexpensive standard technolog is possible and issufficient for many applications. A major challenge will be to identify application areas wherewe can deploy pervasive computing technology in consumer domains without major investmentsin infrastructure. One possibility is to determine where we can add value to legacy systems byadding embedded server technology, like we did with the EIB implementation. Finally, we haveto give device manufacturers a cost efficient option for making their products ready forintegration into a pervasive computing environment without committing to a particular pervasivecomputing technology or middleware. A simple—yet to be standardized—control interface issuch an option. 21
  • 22. 8. BIBILIOGRAPHY:1. US Patent 5,956,487, ―Embedding Web Access Mechanisms in an Appliance for UserInterface Functions Including a Web Server and Web Browser,‖1996, document US 5956487;www.depatisnet.de.2. S. Hartwig et al., ―WAP over Bluetooth: Technology and Applications,‖ IEEE Int’l Conf.Consumer Electronics, IEEE Press, Piscataway, N.J., 2001.3. J. Bentham, TCP/IP Lean, CMP Books, 2000.4. B.A. Myers, ―Using Hand-Held Devices and PCs Together,‖ Comm. ACM, vol. 44, no. 11,Nov. 2001, pp. 34–41.5. J. Burkhardt et al., Pervasive Computing, Addison Wesley, Reading, Mass., 2002.6. Patent Application WO 01/41408 01 A1, ―A Device and a Method for Operating an ElectronicUtility Device from a Portable Telecommunication Apparatus,‖ document W0200141408;www.depatisnet.de.7. T. Kindberg et al., ―People, Places, Things: Web Presence for the Real World,‖ Proc. 3 rd IEEEWorkshop Mobile Computing Systems and Applications (WMCSA’00), IEEE CS Press, LosAlamitos, Calif., 2000, pp. 19–21. 22